Security Trends: Video surveillance technology of wireless transmission (1).

Video surveillance access way by wired to wireless is an inevitable trend.

Video transmission wuxianhua broke the traditional coaxial cables and fiber image monitoring subject to hardware connection unfavorable situation, more flexibility and convenience, based on wireless network video monitoring system came into being. Wireless video transmission technology development has on wireless mobile network architecture and Protocol have had far-reaching consequences, but because of the wireless channel bandwidth resources limited, interference, and the video signal data volume, real-time performance requirements higher.

Wireless video transmission technology development status

With the development of the information society, people security monitoring of increasingly high demands, in addition to centralized government organs, enterprises and institutions, such as in the sea, mountains, mines, basements, and other complex environments to achieve erection of local cable network.

All you need to implement security video surveillance that requires wireless video transmission technology.

At present, the market for wireless video transmission technology mostly used GPRS and CDMA technology.

And GPRS transmission bandwidth, transfer video to only a few frames per second, and an emergency event is prone to breakpoint and wireless receiver in the corner. CDMA transmission also exist such a defect, the downlink bandwidth is 153 kb/s, uplink bandwidth is 70 ~ 80 kb/s, and transmission fluid video basically impossible to achieve. Because the image is only a few frames to capture to transmission, and for the small screen size. Obviously, this will not be able to meet the video surveillance system for real-time applications. For microwave (digital spread spectrum microwave, microwave), wireless LAN (WLAN, 802.11 (a.b, g)), and other techniques of other higher wireless transmission scheme, its realization video encoding to MPEG-2/4, H.264, etc. mainly. But most of them are common problems that can only be achieved through sight transmission, directional transmission, and difficult to support mobile transmission, thus limiting the use of video surveillance systems.

Monitoring systems generally use a low frame rate and guarantee the transmission transmission clarity, because only MPEG-4 CIF above image definition to meet the needs of the investigation.

Therefore, wireless transmission technology to monitor adequately play absolute advantage, should meet: in non-visual and application blocking environment; suitable for high-speed mobile wireless transmission in real time images; suitable for transmission of high-bandwidth, high-definition streaming, high definition video and audio; Excellent anti-jamming ability, resistance to fading. Practical, real-time wireless video transmission there are two main concepts: first mobile transmission; second, broadband transmission. Therefore, we have developed the ability to band wide high-definition video for stabilization of infinite video transmission systems, data transmission mechanism optimization is required to solve one of the key issues, unlimited link bandwidth resources are limited, this limitation is reflected in the massive video to be handed down. Here, for wireless video transmission system data transport mechanism of DVR expand the appropriate studies aimed at addressing wireless video transmission bandwidth resources are limited and video data capacity that conflict, take full advantage of the video signal of the spatio-temporal correlation to save unnecessary retransmissions of bandwidth wasted resources. Through the use of bandwidth a distortion of the concept of cost function to evaluate wireless video transmission system. On this basis, further gives a distorted based bandwidth cost minimization standards section retransmission error control mechanism, thereby improving bandwidth utilization, and the appropriate laboratory analysis.

Wireless video transmission mechanism analysis and fault-tolerant transmission technology

Reliable channel signal transmission on aims to take full advantage of the channel bandwidth resources; and for unreliable channel, transmission of focus is making full use of bandwidth resources to achieve reliable transmission, that is, fault-tolerant transmission technology.

Here in the wireless channel video transmission mechanisms, the main point of research is fault-tolerant transmission control. Fault-tolerant transmission control technology based on how the control can be divided into three main categories: that is, forward error control, based on the feedback of ARQ and source-channel coding. Forward error control (Forward Error Control, FEC) including channel error correction coding technology, cutting packaging technology and optimized packet scheduling mechanism. Based on the feedback of ARQ technologies including the use of multiple frames of reference mechanism of reference frame selection (Reference Picture Selection, RPS) mechanism, hybrid ARQ (Hybrid, HARQ) mechanism and ARQ feedback based bug tracking technology. Due to the fault-tolerant based ARQ transmission control technology with excellent performance, so in this highlight ARQ-related transmission control technology, and discusses existing video-tolerant transmission mechanism exists.

Forward error control use of forward error correction coding way to overcome the channel faults.

Error probability wave channel more strenuous circumstances (such as existing mobile channels), in order to obtain certain transmission quality, forward error correction coding must be based on current estimates of the worst situation to increase redundancy check bit, which will cause the waste of bandwidth. Bandwidth resources already limited wireless channel, is obviously not able to meet the requirement. To do this, consider the ARQ technology and forward error control, called HARQ technology. HARQ is divided into two categories: HARQ I class, the sender of the encoding you want to have a certain error correction capability, when the receiver finds an error, the first use of forward error correction coding to correct the error. If the error is corrected, the transfer to the sending side of a current package receive a success message (ACK), otherwise send receive failure message (NACK). The sender if you receive an ACK, then continue with the next packet sent; otherwise, an error packet be resent. So I kind of ARQ need strong forward error correction coding, error rates lower applications will result in a waste of bandwidth resources, but in error rates high environment to access than any other type ARQ system better throughput efficiency. Ⅱ type ARQ in only requires forward error correction coding with error detection capabilities, based on the channel coding error correction capacity of theory, this could save bandwidth. When the receiver findsError, send a retransmission request; send-side only transfer error data with error correction capacity corresponding to the parity-check codes. When the receiver receives, if you still cannot correct the error, you continue to send ARQ, the sender can choose retransmission overall error data and parity-check codes, you can also choose to send more error correction capability check code, specifically because of the different control strategies can be adjusted. In view of the wireless channel error rate is high, with a feedback channel wireless transmission usually HARQ-I. Figure 2 shows the use of HARQ-I wireless video transmission system, map dotted box represents a transmission error control processes. According to the design principle of HARQ-I, receive-side errors found, first of all for forward error correction (figure in the first layer of error barrier), and if not corrected and the current system meet the delay limit, send ACK request to let send-side upload error section of the data (the second layer error barrier). Such retransmission can be repeated to the receiver receives the correct data or retransmission delay beyond system delay limit. If the retransmission is finished still cannot get the correct packet, the receiver uses the error hiding techniques for error recovery (third layer error barrier). You can see, it's the basic idea is an error after use of ARQ technology to restore errors, so here it is named the "best" ARQ mechanism (BestEffortARQ, BEA, RQ).

As the video signal has the strong correlation between time and space, and the coding side does not completely remove this dependency, makes the decoding side to take advantage of these residues in related to recover a certain quality of video.

Recovery of quality also and be restored part of the texture, and movement are closely related, generally speaking, compared to the texture and movement compared single gentle, recovery effects are better than others. In this case, if you use BEARQ to retransmit this part of the video will result in a waste of bandwidth. [1] [2]